Optical disc media now widely available include CD-R/RW, DVD-RAM, DVD±R/Rw, BD, etc., and some of them have two data layers. Optical disc devices adapted for record and playback with those optical disc media mentioned above, i.e. so-called DVD super multi-drives, are now widely in use. In the near future, high-performance disc drives adapted to blue ray discs (hereafter referred to simply as BDs) will come to be widely used. Further, there's need for optical discs having still higher capacity.
The increase in the drive speed of optical disc drive and in the density of information recorded in the optical disc has necessitated the introduction of a technique in which the binarization of reproduced signal is performed by Partial Response Maximum Likelihood (PRML) reproduction procedure. As one of the PRML procedures is known the adaptive PRML procedure (or system) or compensatory PRML procedure (or system) which can adaptively change the target signal level in accordance with the reproduced signal. The non-patent document 1, “Journal C of Institute of Electronics, Information and Communication Engineers, Vol. J90-C, p. 519 (2007)” discloses the fact that a device adapted to BDs can achieve high-density recording equivalent to 35 GB by using such a PRML procedure wherein the asymmetry of reproduced signal and the thermal interference at the time of recording can be compensated. It is pointed out in the document that reproduction performance under the condition for high-density recording is higher for larger constraint length (bit length representing ‘class’). In order to obtain the best result from binarization, an automatic equalizer that makes minimum the RMS error between reproduced signal and the target signal for the PRML index, is installed in an optical disc device provided with such a PRML procedure. In general, such an automatic equalizer is installed as a Finite Impulse Response (FIR) filter having its tap factors variable.
The increase in the recording density in an optical disc leads to the decrease in the size of each recorded mark as compared with the size of the light spot, and therefore results in the reduction of the amplitude of the obtained reproduced signal. The resolving power of the light spot is determined by the wave length λ and the aperture number NA of the objective lens. Accordingly, if the length of the recorded mark having the shortest run length becomes equal to or less than λ/4NA, the amplitude of the signal corresponding to the repeated series of such shortest recorded marks is reduced to zero. This phenomenon is known as “optical cutoff”, and may occur when λ/4NA≈119 nm in the case of BDs. In the case of BDs, an attempt to attain a recording capacity of more than about 31 GB with the track pitch kept constant, causes the amplitude of the signal corresponding to the repeated series of recorded marks having the shortest run length of 2T to be reduced to zero. It is therefore indispensable to use a PRML procedure in order to acquire a satisfactory reproduction performance under such a high-density condition.
When desired information is recorded in a recordable optical disc, the state of crystallization, for example, in the recording film is changed by using pulsed laser light whose intensity is modulated (hereafter referred to as ‘recording pulse’). Materials for such recording films include phase-variable substances, organic pigments, certain kinds of alloys and oxides, all of which are well known and widely used. In the mark edge coding method for use with CDs, DVDs and BDs, code information is determined depending on the positions of leading and trailing edges. Regarding the recording pulses, the positions and widths of the first pulse that mainly determines the condition for forming the leading edge of a recorded mark and the last pulse that mainly determines the condition for forming the trailing edge of the recorded mark, are important to maintain the quality of recorded information in a good condition. Therefore, it is customary with recordable optical discs to use “adaptive recording pulses” which can adaptively change the positions or widths of the first and last pulses in accordance with the length of each recorded mark and the lengths of the spaces that immediately precedes and follows the recorded mark.
Under such a high-density recording condition as described above, since the size of each formed recorded mark becomes very small, it is necessary to choose the condition for radiating the recording pulses (hereafter referred to as “recording condition”) with a higher precision than conventional. On the other hand, in an optical disc device, the shape of the light spot varies depending on the wavelength at the light source, wave front aberration, focusing condition, the tilt of disc, etc. Further, since the ambient temperature and the aging effect change the impedance and the quantum efficiency of the semiconductor laser device, the shapes of the recoiling pulses change accordingly. The technique for invariably obtaining the best recording condition in response to the shapes of light spots and the shapes of the recording pulses both of which fluctuate depending on environments and devices, is usually called “test writing”. Such a technique for adjusting the recording condition by using the test writing will become more and more important with the requirement for further increasing recording density.
Adjusting techniques for recording condition are classified roughly into two categories: one method uses bit error rate or byte error rate as index and the other utilizes statistical index such as jitter. The former pays attention to an event that occurs with a small probability with respect to recorded data and the latter is concerned with the average quality of recorded data. Regarding write-once optical discs, for example, in the case where data are recorded in and reproduced from plural locations in the disc with the recording condition varied, even the best recording condition for the former method may cause a large bit error or byte error if fingerprints overlie the recorded data. Therefore, the former method should not be selected in this case. The best recording condition should be such that the average quality of the data recorded under such a recording condition is optimal. It can therefore be said that the method using statistical index is preferable for storage media such as optical discs, which are vulnerable to material flaws, fingerprints or dust.
Methods corresponding to PRML procedure for statistically evaluating the quality of recorded data are disclosed in, for example, the non-patent document 2, “Jpn. J. Appl. Phys. Vol. 43, p. 4850 (2004)”; the patent document 1, JP-A-2003-141823”; the patent document 2, “JP-A-2005-346897”; the patent document 3, “JP-A-2005-196964”; the patent document 4, “JP-A-2004-253114”; and the patent document 5, “JP-A-2003-151219”.
The patent document 1 discloses the technique wherein use is made of the certainty Pa corresponding to the most likelihood state shift array and the certainty Pb corresponding to the secondary likelihood state shift array so that the quality of reproduced signal is evaluated on the basis of the distribution of |Pa−Pb|. The non-patent document 2 discloses a technique wherein the value obtained by subtracting the Euclidean distance between two target signals from the absolute value of the difference between the Euclidean distance (corresponding to Pa) between the target signal representing the binary bit array (corresponding to the most likelihood state shift array) derived from the reproduced signal and the reproduced signal, and the Euclidean distance (corresponding to Pb) between the target signal representing the binary bit array (corresponding to the secondary likelihood state shift array) derived through a single-bit shift of the interested edge and the reproduced signal, is defined as MLSE (Maximum Likelihood Sequence Error), and the recording condition is adjusted in such a manner that the average value of the distribution of MLSEs is reduced to zero for every recorded pattern.
The patent document 2 discloses a technique wherein edge shift is specifically noted: a virtual pattern having a run length of 1T is used as an error pattern for showing that the edge of reproduced signal shifts to the right or left; the amount of edge shift is obtained by calculating the difference between sequence errors having plus or minus sign depending on the direction in which the edge shift occurred; and the recording condition is so adjusted as to cause the amount of edge shift to approach zero. In this case, the evaluating index is called “V-SEAT (Virtual state based Sequence Error for Adaptive Target) index”.
The patent documents 3 and 4 disclose a technique wherein the difference between the Euclidean distance between reproduced signal and correct pattern and the Euclidean distance between reproduced signal and error pattern, is calculated by using a table containing the combinations of correct patterns and error patterns corresponding the correct patterns; and the Simulated bit Error Rate (SbER) is obtained from the average and standard deviation of the Euclidean distance differences.
The patent document 5 discloses a technique wherein, on the basis of the difference between the Euclidean distance between reproduced signal and correct pattern and the Euclidean distance between reproduced signal and error pattern, the error probabilities corresponding respectively to the case where the interested edge has shifted to the left and to the case where it has shifted to the right, are obtained; and the recording condition is so adjusted as to make the probabilities corresponding to the two cases equal to each other. Accordingly, use is made of a preselected reproduced signal, a first pattern whose wave pattern corresponds to that of the preselected reproduced signal, and an arbitrary pattern (a second or a third pattern) whose wave pattern corresponds to that of the preselected reproduced signal but which is different from the first pattern. First, the distance difference D=Ee−Eo between the distance Eo between the reproduced signal and the first pattern and the distance Ee between the reproduced signal and the arbitrary pattern, is obtained. Secondly, the distribution of the distance differences Ds with respect to plural samples of reproduced signals is obtained. Thirdly, the quality evaluation parameter (M/σ) is determined on the basis of the ratio of the average M of the obtained distance differences Ds to the standard deviation σ of the obtained distribution of the distance differences Ds. And finally, the quality of reproduced signal is assessed from the evaluation index value (Mgn) represented by the quality evaluation parameters.